Fuel cell

ABSTRACT

The invention aims at providing a platinum black material, without using an expensive and rare material, which is excellent in CO poisoning inhibiting effect, H 2 S poisoning inhibiting effect, SO 4  poisoning inhibiting effect and HCHO poisoning inhibiting effect, and a method for fluorinating platinum black. The platinum black material is characterized by fluorine adsorbed on its surface. The method for fluorinating platinum black is characterized by allowing platinum black to stand in a mixed gas atmosphere of n inert gas and fluorine in a low-pressure chamber to make fluorine adsorbed on the surface of the platinum black.

TECHNICAL FIELD

The present invention relates to a platinum black material, a method forfluorinating platinum black, an electrode, a single-sided membraneelectrode assembly (MEA), and a polymer electrolyte fuel cell.

BACKGROUND ART

-   Patent Document 1: Japanese Unexamined Patent Publication (Kokai)    No. 2002-159866-   Patent Document 2: Japanese Patent Gazette No. 3350691

Polymer Electrolyte Fuel Cells (PEFCs) are devices, which are capable ofdirectly converting chemical energy into electrical energy, startingfrom room temperature, being downsized and lightened; and are expectedto be used as new energy sources for such applications as co-generationfor households and automobiles. Among the methods for supplyinghydrogen, the fuel necessary for PEFCs, hydrocarbon reforming isconsidered to be the easiest approach considering the infrastructuredevelopment; however, the Pt catalysts on the PEFC anodes adsorb the COin the reformed gas, or are poisoned by the gas, and are deactivated,making their practical applications difficult.

In addition, gases other than CO, such as H₂S, SO₄ and HCHO, aresuspected to lower the catalyst's capabilities. As a countermeasure,Pt—Ru alloys are currently used as catalysts with a tolerance to COpoisoning (Patent document 1). However, Ru is a rare metal and isanticipated to become a huge bottleneck in the future progress of thePEFCs because of risks, such as a steep rise of its price. Therefore,one of the most important issues, in the development of PEFCs, is todevelop a catalyst with a tolerance to CO poisoning, which takes theplace of the Pt—Ru (Patent document 1).

As a technology for a catalyst with a tolerance to CO poisoning, atechnology for surface treatment (fluorination) of a hydrogen absorbingalloy is described in Patent Document 2. The fluorination inhibits aLaNi₅ hydrogen absorbing alloy, which, as in the case of PEFC anodes,suffers from being deactivated by CO adsorption, from being poisonedwith CO, in this case, by having fluorine adsorbed on the catalyst'ssurface either from a diluted fluorine gas, or from hydrogen fluoride;the fluorination inhibits the catalyst from being poisoned with CO.Because the method is featured by a capability of changing thecatalyst's characteristics after the preparation of the catalyst, acapability which did not exist in the conventional catalyst preparationmethods, the method is expected to provide, if brought into practice, aneasy-to-use inexpensive catalyst preparation.

However, the technology described in the Patent Document 2 involvesforming a film, mainly comprising a metal fluoride, on a hydrogenabsorbing alloy; but metal fluorides do not necessarily inhibit othermetals, or other alloys, from being poisoned with CO.

Furthermore, there is a need for a material excellent not only in atolerance to CO poisoning, but also in tolerances to H₂S poisoning, SO₄poisoning and HCHO poisoning.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

The invention aims at providing, without using an expensive and rarematerial such as Ru, a platinum black material excellent not only in atolerance to CO poisoning, but also in tolerances to H₂S poisoning, SO₄poisoning and HCHO poisoning, and a method for fluorinating platinumblack.

Means for Solving Problem

One embodiment is a platinum black material comprising a surfaceprocessed by a treatment including contact with fluorine gas or with amixed gas of an inert gas and fluorine gas.

A further embodiment is a platinum black material comprising fluorinepresent on the surface in an atomic monolayer.

The surface may be pretreated to remove impurities prior to thetreatment. Incidentally, the term “impurities” used herein indicatesimpurities such as water in gaseous or liquid form, oxygen or oxides.

A yet further embodiment is a method for fluorinating platinum black,comprising a step of allowing platinum black to stand in a fluorine gasatmosphere, or in a mixed gas atmosphere of an inert gas and fluorine,in a low-pressure chamber, and a further step of allowing the platinumblack to stand in a vacuum of 1 Pa or lower in order to degasify thesurface.

The fluorine concentration in the mixed gas may be 0.001% or higher.

The method may include a step of pretreating the platinum black toremove impurities present on its surface prior to the fluorination.

The fluorination may be carried out in a heating condition.

In the heating condition the heating temperature may be 20° C. orhigher.

In the method, the inert gas comprises at least one of nitrogen, He andAr.

In a further embodiment, the platinum black material treated by theabove-described method.

The platinum black material has excellent tolerances to CO poisoning,H₂S poisoning, SO₄ poisoning and HCHO poisoning.

A yet further embodiment is an electrode comprising the above-describedplatinum black material.

A still further embodiment is a single-sided membrane electrode assemblycomprising the just-noted electrode supported on one side of theassembly.

Another embodiment is a polymer electrolyte fuel cell comprising thejust-noted single-sided membrane electrode assembly.

Effect of the Invention

The following effects are achieved by the platinum black according tothe present invention. Expensive and rare materials are not required.

Excellent CO poisoning inhibiting effect is expected. And excellent H₂Spoisoning inhibiting effect, SO₄ poisoning inhibiting effect and HCHOpoisoning inhibiting effect are also expected. The invention is usedsuitably for a PEFC anode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the structural concept of a PEFC half-cell;

FIG. 2 is a graph, in association with an embodiment, showing hydrogenoxidation current in hydrogen gas containing CO;

FIG. 3 is a graph, in association with an embodiment, showing the CVmeasured after the measurement of hydrogen oxidation current, withatmosphere replaced by Ar, at a sweep rate of 10 mV/S; and

FIG. 4 is a graph, in association with an embodiment, showing the XPSmeasurement result of each sample.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, fluorine is adsorbed on the surfaceof platinum black by allowing the platinum black to stand in a mixed gasatmosphere of an inert gas and fluorine in a low-pressure chamber.

(Starting Material: Platinum Black)

Any existing platinum black may be used as the starting material. Forexample, platinum black manufactured by TANAKA KIKINZOKU may be used.

(Low-Pressure Chamber/Pretreatment)

Preferably, impurities are removed from the surface of the platinumblack before fluorination.

The impurities include, for example, water in gaseous or liquid form,oxygen or oxides. Water in gaseous or liquid form or oxygen may beremoved by allowing platinum black to stand in a low-pressure chamber.It is also effective to flow a purge gas.

The degasification is carried out more effectively by treating with heatin addition to reduced pressure in the low-pressure chamber. Replacingthe inert gas several times is also effective.

After placing platinum black in a low-pressure chamber, the pressureinside the chamber is reduced. The vacuum is preferably 1 Pa or lower.This further reduces the adsorbed gas and facilitates the treatment.

In addition, it is preferable to lower the impurity concentrations(especially water concentration) in the low-pressure chamber as much aspossible at the time of the pretreatment to remove the impuritiespresent on the surface. Preferably, the concentration is 100 ppt orlower and, more preferably, 10 ppb or lower.

Oxide impurities may be removed by dissolving them in hydrofluoric acidand separating them by filtration. After the removal, it is preferableto fluorinate without exposing to atmosphere.

(Fluorination)

The fluorination, according to the present invention, is carried out byintroducing a fluorine containing gas into a low-pressure chamber. Thefluorination is a treatment to induce fluorine adsorption on the surfaceof platinum black.

Although depending on temperature and pressure, the concentration offluorine preferably, is from 0.001 to 10% and, more preferably, from0.01 to 1%, in order to induce fluorine adsorption.

A fluorine concentration in the above range results in the adsorption offluorine without the formation of fluoride films.

An elevated temperature is preferable for the temperature offluorination. In particular, the temperature, preferably, is from 0 to300° C. and, more preferably, from 30 to 250° C. This may be carried outby heating the gas.

(Posttreatment)

The following posttreatment is preferably performed after thefluorination described above.

The posttreatment is a treatment to remove excessive fluorine from thesurface. In particular, it is a treatment of degasifying the surface.The surface is degasified so that an atomic monolayer of fluorineremains on the surface. This prevents excessive fluorine fromdecomposing and affecting the surface later.

The degasification for leaving an atomic monolayer of fluorine mainlyinvolves controlling the surface temperature, vacuum and degasificationtreatment time. For example, the fluorine layer on the surface isdesorbed by allowing the surface to stand in a vacuum of 1 Pa or lowerfor 1 hour. However, the fluorine (atomic monolayer) adhered directly toor adsorbed on the platinum black surface is not removed by thetreatment of allowing the surface to stand in the low-pressure chamberunder the prescribed pressure for the prescribed period of time. Thesurface may be kept in the low-pressure chamber for 1 hour or longer. Inparticular, the conditions for the atomic monolayer remaining may bepredetermined through actual experiments, in which platinum black istreated for fluorination under varying surface temperature, vacuum, anddegasification time.

(Fluorine Adsorbed Platinum Black)

Fluorine is adsorbed on the surface of platinum black by thefluorination treatment described above. Fluorine should be adsorbedwithout forming platinum fluoride and, preferably, adsorbed to form anatomic monolayer.

Example 1

An experiment was carried out using platinum black manufactured byTANAKA KIKINZOKU.

(Pretreatment)

A pretreatment was carried out under the following conditions.

The treatment was carried out by repeatedly introducing and evacuatingN₂ to and from a chamber under a reduced pressure for several times andsubsequently introducing N₂ at 100° C. for 2 hours.

(Fluorination)

The pretreated platinum black was placed in the chamber evacuated to avacuum pressure of 1 Pa or lower, and fluorine was adsorbed by allowingthe platinum black to stand in a mixed gas of N₂ and F₂ with adjustedvolume concentration of fluorine for 3 hours.

The weight difference was measured at this time to confirm fluorineadsorption on the platinum black. The mixed-gas temperature was raisedto 250° C. to perform the fluorination.

(Posttreatment)

After the fluorination, the chamber was evacuated to 1 Pa or lower andheld at the pressure for 1 hour to remove fluorine adsorbed excessively,and the sample was retrieved quickly under N₂ atmosphere.

The catalyst sample thus prepared was mixed with 5 wt % Nafion® solutionand pure water to form a catalyst ink, which then was supported on acarbon paper (TGP-H-090 manufactured by Toray) of φ=10.0 mm with 300μg/cm². After drying at room temperature, the above was joined to aNafion 115 (manufactured by Dupont) membrane by hot pressing at 135° C.for 3 minutes to form a single-sided MEA.

A gas passage was made on the MEA holder to allow supply of gas to thediffusion layer of the single-sided MEA thus prepared, which was put incontact with a collector or a working electrode to configure a half-cellas shown in FIG. 1.

The reference electrode was an RHE and the counter electrode was aplatinum plate. The electrolyte was 1M perchloric acid, which was heatedto 50° C. using an immersion heater, to reproduce PEFC operatingconditions as much as possible. When hydrogen oxidation occurs on theworking electrode, hydrogen is generated on the counter electrode, whichmay disturb the reaction at the counter electrode and, in order to trapthis, Ar gas was introduced in bubbles into the solution.

Ar gas was first introduced into the diffusion layer of the single-sidedMEA and cyclic voltammogram (CV) was measured at an electric potentialof 50 to 1500 mVvsRHE and sweep rate of 100 mV/S. Then, the cleaning ofthe electrode surface and the background measurement were performed.Subsequently, a hydrogen gases containing 0 ppm, 50 ppm, and 100 ppm ofCO were introduced, 50 mVvsRHE was applied for 40 min in the mixed gasto stabilize the CO adsorption on the electrode, and hydrogen oxidationcurrent was measured within the electric potential range of 50 to 300VvsRHE at the sweep rate of 0.5 mV/S to measure the effect of CO on thehydrogen oxidation.

(Fluorinated Pt Black Catalyst)

FIG. 2 shows the result of comparison of the hydrogen oxidation current,measured in a hydrogen gas containing 100 ppm of CO, for Pt blackelectrodes fluorinated in different manners. The results indicate thatthe Pt black electrodes, fluorinated with 1% and 10% concentration,exhibit currents higher than the untreated one for both the COconcentrations of 50 ppm and 100 ppm with the one treated with 1%exhibiting more enhanced effect.

On the other hand, the hydrogen oxidation current of the sample treatedwith 10% fluorine concentration declines to 70.2% of the untreatedplatinum black in pure hydrogen atmosphere, indicating that the hydrogenoxidation is disturbed, along with the reduction in the surface area ofwave of hydrogen in Ar atmosphere. (Table 1)

TABLE 1 Comparison of H2 oxidation current for fluorinated Pt blackcatalyst in this study H₂ Oxidation Current True surface area/ F₂Current Apparent surface area (concentration) density/mAcm^(−2a))(cm²/cm²) 0% 470 68.9 1% 469 69.5 10%  349 14.2 ^(a))200 mV vs. RHE

After the hydrogen oxidation current was measured at CO concentration of100 ppm, the atmosphere was replaced by Ar and CVs were measured at asweep rate of 10 mV/S: it was indicated that both the untreated Pt blackand the Pt black fluorinated by 10% concentration did not exhibit anyregion of wave of hydrogen as a result from CO poisoning; however, thePt black fluorinated by 1% concentration exhibited the region of wave ofhydrogen: and the results show that the Pt black fluorinated by 1%concentration has CO poisoning inhibiting effect. (FIG. 3)

FIG. 4 shows the results of XPS measurements on each sample. Almost noshift was observed in the Pt4f wave form of the Pt black treated with 1%concentration, while a remarkable shift was observed in the Pt4f waveform of the Pt black treated with 10% concentration, along with its F1speak shifted toward the lower energy side compared to the Pt blacktreated with 1% concentration. The study by E. Bechtold et al., in whichfluorine was adsorbed onto the surface of a Pt single crystal in a highvacuum, reported that fluorine is present in the form of a monolayeradsorbed on the surface of the Pt when a small amount of fluorine wasadsorbed, while fluorine is present in the form of a Pt fluoride, PtF₄,when a large amount of fluorine was adsorbed. Assuming that the samephenomena occur on Pt polycrystal surfaces, the difference, inelectrochemical behavior between different fluorine concentrations, isconjectured that fluorine is adsorbed as a monolayer onto Pt in the caseof 1% fluorine concentration, while PtF₄ is formed in the case of 10%fluorine concentration.

One mechanism among others, has been proposed for hydrogen absorbingalloys, that fluorides such as LaF₃ formed on its surface inhibitsadsorption of impurities such as CO. On the other hand, in the case ofPt catalysts, it was shown that the formation of a fluoride inhibitshydrogen oxidation. The inhibition is considered to be due to the factthat, unlike hydrogen absorbing alloys, the reactions on the anode Ptcatalysts require the formation of three-phase interfaces, in whicheffective surface area for Pt activation is reduced by the formation ofthe fluoride.

The result exhibited a remarkable effect in the case that the fluorineconcentration for fluorination was 1%, while hydrogen oxidation wasinhibited in the case that the fluorine concentration was 10%. This isconsidered to be due to the fact that fluorine may exist on the surfaceof platinum in two different forms, i.e., fluorine monolayer adsorptionand platinum fluoride.

The tolerances to H₂S poisoning, SO₄ poisoning and HCHO poisoning werealso measured. The results showed that all the tolerances were superior.

INDUSTRIAL APPLICABILITY

The platinum black according to the present invention is excellent in COpoisoning inhibiting effect, H₂S poisoning inhibiting effect, SO₄poisoning inhibiting effect and HCHO poisoning inhibiting effect, and issuitable for use with electrodes and, in particular, the electrodes forfuel cells.

The fuel cell according to the present invention may use the COcontaining hydrogen gas, obtained from hydrocarbon reformation, as itsfuel gas.

As a result, supply of fuel is made possible without furtherinfrastructure development and is used as energy sources for,especially, automobiles and co-generation for house hold.

1. A method for fluorinating platinum black, comprising a step ofallowing platinum black to stand in a fluorine gas atmosphere, or in amixed gas atmosphere of an inert gas and fluorine, in a low-pressurechamber and a further step of evacuating the chamber to a vacuum of 1 Paor lower and allowing the platinum black to stand in the vacuum in orderto degasify a surface thereof.
 2. The method for fluorinating platinumblack according to claim 1, wherein the fluorine concentration in themixed gas is 0.001% by volume or higher.
 3. The method for fluorinatingplatinum black according to claim 2, further comprising a step ofpretreating the platinum black to remove impurities present on itssurface prior to the fluorination.
 4. The method for fluorinatingplatinum black according to claim 3, wherein the fluorination is carriedout in a heating condition.
 5. The method for fluorinating platinumblack according to claim 3, wherein the inert gas comprises at least oneof nitrogen, He and Ar.
 6. The method for fluorinating platinum blackaccording to claim 2, wherein the fluorination is carried out in aheating condition.
 7. The method for fluorinating platinum blackaccording to claim 2, wherein the inert gas comprises at least one ofnitrogen, He and Ar.
 8. The method for fluorinating platinum blackaccording to claim 1, wherein the fluorination is carried out in aheating condition.
 9. The method for fluorinating platinum blackaccording to claim 8, wherein the heating temperature is 20° C. orhigher.
 10. The method for fluorinating platinum black according toclaim 9, wherein the inert gas comprises at least one of nitrogen, Heand Ar.
 11. The method for fluorinating platinum black according toclaim 8, wherein the inert gas comprises at least one of nitrogen, Heand Ar.
 12. The method for fluorinating platinum black according toclaim 1, wherein the inert gas comprises at least one of nitrogen, Heand Ar.
 13. The method for fluorinating platinum black according toclaim 1, wherein the platinum black stands in the vacuum in order todegasify a surface thereof for at least one hour.